| A proton-sucrose symporter mediates the key transport step in plants that utilize apoplastic phloem loading. Sucrose transport activity and steady-state mRNA levels of a sugar beet leaf sucrose symporter ( BνSUT1) are negatively regulated by sucrose in a system that may act to regulate phloem loading. In situ hybridization was used to determine the cell-specific expression pattern of BνSUT1 . This symporter gene was expressed solely in phloem companion cells of minor and higher-order veins, consistent with a role for in phloem loading of sucrose.; Kinetic analysis of sucrose transport suggested that the observed change in activity could be caused by decreased BvSUT1 protein abundance. Protein abundance and transcriptional activity were measured in the presence and absence of sucrose. Immunoblots showed that BvSUT1 protein abundance decreased in a sucrose concentration-dependent manner, and the decrease was proportional to the change in sucrose transport activity measured in purified plasma membrane vesicles. Protein synthesis inhibitors showed that the symporter turned over rapidly. Furthermore, nuclear run-on experiments showed that BνSUT1 transcription was depressed by sucrose, while transcriptional inhibitors showed that symporter message turned over rapidly in the presence or absence of sucrose. These data support the hypothesis that phloem loading is regulated by sucrose symporter protein abundance, which is directly proportional to transcriptional activity.; Potential sucrose-mediated regulation of three leaf-expressed Arabidopsis sucrose symporters was examined using RT-PCR. Expression of AtSUC2 was repressed by sucrose, hexoses, and phenyl-glucoside sucrose analogs, but not by 2-deoxyglucose, 3-O-methylglucose, KCl, or sorbitol. AtSUT2/SUC3 was enhanced by sucrose, sorbitol, and KCl, but not by hexoses, 2-deoxyglucose, 3- O-methylglucose, or sucrose analogs. AtSUT4 was stimulated by sucrose, hexoses, sorbitol, and KCl, but not by 2-deoxyglucose or 3-O-methylglucose. Interestingly, a phenyl-glucoside sucrose analog repressed AtSUT4 expression.; Taken together, the evidence supports the hypothesis that a sucrose-mediated regulatory pathway common to at least two plant families acts as a key step in regulating assimilate partitioning by modulating phloem loading in response to changing levels of sink demand. Also, symporters that are members of two discovered sucrose transporter families are regulated by osmotic potential in a novel pathway that may be involved plant responses to water stress. |
